In a conventional filling machine for filling a can with a beverage, such as beer or soft drinks, the cans are fed by a star wheel conveyor to the filling machine and individually positioned on a surface of a rotating turntable beneath a filling head. With the can positioned on the surface, a cylindrical tulip is moved downwardly and seals against the upper edge of the can.
The conventional filling machine also includes a bowl or reservoir which is spaced above the supporting surface and contains the liquid or beverage while the headspace above the liquid contains carbon dioxide.
In the typical filling operation, the can which is sealed against the tulip is initially purged with carbon dioxide from the bowl for a predetermined time period to flush air from the can. A vent tube and a filler tube are mounted concentrically within the tulip, and after purging with carbon dioxide, liquid is introduced from the bowl into the can while the gas from the can is vented into the headspace of the bowl through the vent tube. The liquid will rise in the vent tube to the liquid level of the bowl, and in the conventional filling machine the liquid is blown from the vent tube using gas from the filler bowl headspace. The gas is then released from the headspace of the can to the atmosphere by an operation commonly referred to as "snifing".
The conventional filling machine has certain drawbacks. The carbon dioxide which is used to purge the cans is not pure carbon dioxide but is contaminated with air or oxygen due to the fact that the gas vented to the headspace of the bowl contains some air and the headspace gas is then used for purging during the next cycle. The oxygen contaminant in the carbon dioxide can could cause flavor deterioration of the liquid or beverage being canned.
A further disadvantage of the conventional filling process is that the liquid drawn upwardly within the vent tube is removed by blowing gas from the filling bowl through the vent tube into the can, or alternately, blowing out the vent tube after the can has been removed from the filling head. In the former situation, the blowing of the vent tube with the headspace gas from the bowl can cause foaming of the product as well as incorporation of gas within the product, while blowing the vent tube after the can has been removed from the filling head results in product loss.
In the conventional filling machine, the vent tube is fixed to the tulip with the result that the tulip, by necessity, has a relatively long stroke of movement which adversely affects the overall speed of the filling operation.
As the gas in the filling bowl is used to pressurize the can in the conventional machine, the liquid level in the bowl is critical and determines the fill height of liquid within the can.
With the conventional filling machine there is a slight amount of residual liquid remaining between the valve or screen and the lower end of the filling nozzle which is commonly referred to as "after run". While the "after run" is normally within fill tolerances, the liquid level cannot be precisely controlled due to the "after run".